(1) Field of the invention
The present invention relates to a system and method for detecting engine knock in an internal combustion engine and particularly to a system and method for detecting engine knock on the basis of an integrated value of a knock signal derived from a knock sensor.
(2) Background of the art
Various types of ignition timing control systems have been proposed in which knock sensor(s) are installed on the engine to detect and inform of the occurrence of engine knock vibrations, the ignition timing is fed back to the limit of the knock frequency on the basis of the signal from the knock sensor(s).
A Japanese Patent Application First publication No. Showa 52-87537 published on July 21, 1977 exemplifies one of the previously proposed ignition timing systems in which during a constant interval after ignition of any engine cylinder is carried out, the knock signal is integrated and a control unit determines occurrence of knock when the integrated value of the signal thereof exceeds a reference value. A method of determining knock occurence is also exemplified by U.S. Pat. No. 4,640,249 issued on Feb. 2, 1987.
Specifically, after the output signal of the knock sensor is supplied to a band pass filter to pass the knock frequency component therethrough, the knocking frequency is rectified and integrated during a constant crank interval, e.g., ATDC (After Top Dead Center) 10.degree. through ATDC 60.degree.. The derived integrated value indicates the knock energy for each ignition.
Therefore, more accurate knock determination can be carried out as compared with determining knock from a peak value of the output signal from a knock sensor as in previous methods of knock detection known in the art.
Since engine knock is often generated at slightly different timings due to uncertain factors such as fuel use characteristics, it is desirable to provide a relatively long period for the integration interval in order to make an accurate determination of engine knock.
However, in multi-cylinder engines such as an eight-cylinder or twelve-cylinder engine, the interval between ignition of each engine cylinder becomes short. Therefore, if the integration interval is provided as, for example, ATDC 10.degree. through ATDC 60.degree., there is a possiblity that ignition of the subsequent cylinder will be executed within the integration interval provided for the previous cylinder when the ignition timing is advanced in a high-speed, low-load running state of the engine. Consequently, erroneous detection of knock due to ignition noise may result.